Wouldn't necessary mean custom PCB, could just use double-density RAM modules than the 2gb version uses.
What is the EVGA product code for the card? Check it against CoolingConfigurator.com
Edit: There is only one EVGA 680 Classified 4GB on that site (non-reference PCB) and looks like EK isn't making a full-cover block for it, but there is a 2gb version that is reference PCB.
I think you are out of luck finding a block for that card unless EVGA has a Hydrocopper version you can buy.